Method for producing composite material mainly composed of carbon and boron

ABSTRACT

An oxidation resistant carbon material including a composite material is provided. The composite material is obtained by impregnating a carbon material with a boron oxide and/or a hydrate compound thereof, and baking such carbon material under pressure by inert gas at a temperature of not lower than 1500° C.

This application is a division of pending application Ser. No.08/104,410 filed Aug. 10, 1993, which is a continuation of applicationSer. No. 07/729,723 filed Jul. 15, 1991, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite material of carbon-boron(hereinafter referred to as C-B) and, more particularly, to a method forproducing a composite material in which boron component (hereinafterreferred to as B) is evenly dispersed or distributed in the form ofultrafine particles in carbon component (hereinafter referred to as C)and to various uses of the composite material obtained by the method, inparticular, uses as a neutron absorbent and as an oxidation resistantcarbon material.

2. Description of Prior Art

C-B composite material has been heretofore popularly researched,developed and used as a neutron absorbent in atomic industry, as acarbon material having oxidation resistance and as a structural materialfor machines.

A conventionally employed method for producing the C-B compositematerial comprises the steps of mixing a B₄ C (boron carbide) producedseparately with either a carbon material or a material possible to becarbonized, baking them at high temperature, and treating them to betransformed into a solid solution, as is disclosed in Japanese Laid-OpenPatent Publication (unexamined) No. 1987-108767, Japanese PatentApplication No. 1987-297207, etc.

In the C-B composite material obtained according to the mentionedconventional process, B₄ C is mixed with carbon after crushing andgrinding a large and rough solid of B₄ C. In this respect, because suchgrinding is mechanically performed, there is a limit in the pulverizingof B₄ C. Accordingly, in a C-B composite material thus obtained, thereare a B₄ C portion, a solid solution portion of B₄ C and carbon, and aportion of carbon alone. In effect, when observing microscopically, theobtained C-B composite material is not always completely even as awhole.

In the mentioned conventional method, because the two kinds of powderare mixed with each other, molded and baked, there arises a difficultyin the steps of cutting and machining the baked material, hence aproblem exists from an economical point of view in that materialproductivity of expensive B₄ C is considerably reduced by removal andwaste of cut powder.

Moreover, a most serious problem in the conventional method exists inthat a large amount of mineral impurities other than boron are included(usually 5000 ppm). Those impurities come mixed into the compositematerial due to contact with machines and equipment made of steelcontinuously during each step of grinding, mixing, molding and baking ofcompound material. This disadvantage is very difficult to overcome inthe conventional methods. Coexistence of a small amount of mineralimpurities may not be a serious problem depending upon the use of B-Ccomposite material. However, when employing such a B-C compositematerial mixed with mineral impurities as a material for atomicindustry, the mixed mineral impurities generate simultaneously reactionssuch as isotope transformation, splitting, etc. by radiation exposure orthe like. Accordingly, when using it as a nuclear fusion material, afatal disadvantage of decreasing plasma temperature due to hightemperature elements emitted from the impurities will come out.Therefore, as a carbon material for use in atomic industry, a highlypurified specific material, i.e. an ultrahighly pure material of whichmineral impurity is not more than 5 ppm, more preferably not more than 2ppm, substantially 0 ppm (measured by atomic absorption photometer) isusually employed. In a method for processing such an ultrahigh purematerial, the mineral impurities are eliminated by halogenationtreatment of high volatility, as is disclosed in the Japanese PatentApplication No. 61-224131 for example. In the composite material of B₄C-C, however, because of existence of boron in carbon material,impurities cannot be eliminated by using the mentioned treatment.

The mentioned disadvantage of the conventional method may result in astill further problem in that when using crucible for melting alloy,stirring bars etc. necessary to obtain a specifically preciouscomposition in metallurgical industry and the mentioned instruments areoxidated and worn out, then impurities may be mixed into the alloyeventually resulting in pollution.

Also when using the mentioned composite material as a member to performhot press for baking ceramics, the same problem of pollution asmentioned above may come out.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to solve theabove-discussed problems of the conventional method when using B₄ C-Cpowder as a material and provide a superior B-C composite material.

Another object of the invention is to provide a novel use of the B-Ccomposite material.

To accomplish the foregoing objects, the inventors have been engaged inresearch and development of superior composite material and, in thefirst place, adopted a method for impregnating boron oxide or hydratedcompound into carbon material in the form of melt or solution. By thismethod, it is now possible for boron component to permeate into finecarbon particle surface or fine pores in carbon material at themolecular level so as to be dispersed very finely throughout the entirecarbon material as compared with the use of B₄ C fine particles.

Further, to use boron in the form of an oxide, a reaction methodsuitable for fixing boron into carbon by reaction between the oxide andcarbon and suitable for solid solution treatment is also provided by theinvention. In other words, the inventors have succeeded in developing amethod comprising the steps of impregnating a liquid boron compound intoa carbon material, and baking a mixture thus obtained at hightemperature under high pressure so that the impregnated boron compoundis not volatilized from the carbon material and that boron reactssufficiently on carbon.

It has been recognized by the inventors that the novel C-B compositematerial obtained by the mentioned method performs an excellent functionas a neutron absorbent and that this composite material exhibits afunction of remarkably high oxidation resistance as compared with theconventional B₄ C powder mixing method. The composite material accordingto the invention not only overcomes such disadvantages of the prior artthat, because of the use at high temperature under oxidation atmosphereresulting in oxidation wear, relative members or parts must have beenfrequently replaced, but also exhibits that the composite material issuperior in oxidation resistance and satisfiable in machinability.

Described hereinafter is an illustrative reaction between boron oxide(B₂ O₃) and carbon.

First Process

A molten boron oxide is impregnated into a cut compact of isotropic highconcentration carbon material ("IG-11" produced by Toyo Tanso), forexample, at 600° to 1400° C., more preferably at 800° to 1200° C. underpressure in a pressure container. In this case, it is preferable thatthe impregnation is carried out after once reducing pressure in thecontainer so as to eliminate air out of fine pores of the carbonmaterial, but such preliminarily degassing is not always necessary.

For impregnating B₂ O₃ into the carbon material, several kg/cm² ofpressure is needed, but 50 to 100 kg/cm² is preferable in order toimpregnate B₂ O₃ under pressure completely into the deep portion ofcarbon material. Required pressure value should be appropriately decideddepending upon percentage of voids, particle size, distribution ofpores, temperature, etc.

Second Process

The carbon material impregnated with B₂ O₃ is then subject to heattreatment (hereinafter referred to as HIP) at high temperature underhigh pressure using inert gas as a pressure medium. As a result of thisheat treatment using an inert gas such as Ar as a medium, a pressure isapplied to the carbon material and B₂ O₃ solution evenly from everydirection as if pressure was hydraulically applied to them, therebyconfining B₂ O₃ into the carbon material while preventing evaporationthereof, thus chemical reaction between carbon and boron proceeds.

The pressure and temperature in the heat-treating apparatus is notrespectively lower than 100 kg/cm² and 1500° C., preferably not lowerthan 2000° C. and 1500 to 2000 kg/cm². If the temperature is over 2300°C., undesirable reaction of decomposing the solid solution of carbon andboron may take place simultaneously.

The foregoing first and second processes are essential, and a followingthird process may be added to eliminate a small amount of B₂ O₃ stillremaining in the carbon material.

Third Process

The composite material after completing the HIP treatment in theforegoing second process is further treated under the pressure nothigher than 10 Torr, preferably not higher than 5 Torr, and at thetemperature not lower than 1000° C., preferably not lower than 1500° C.,whereby quantity of B₂ O₃ in the composite material is decreased to notmore than 0.1%. In the B-C composite material thus obtained, boron isvery finely and evenly dispersed and distributed throughout thecomposite material as compared with the conventional material employingB₄ C powder.

In the invention, any carbon material including common carbon material,aerotropic carbon material, carbon-carbon composite material(hereinafter referred to as C/C material) in addition to the mentionedisotropic carbon material can be employed irrespective of kind thereof.Since carbon powder is not used in the invention, carbon can beboronized without changing a given shape, organization and structure ofthe carbon composite, i.e., as they are which is one of the mostimportant advantages of the present invention.

When the mentioned boronization is carried out using an ultrahigh pureisotropic high density graphite as a matrix for example, quantity ofimpurities other than carbon and boron elements of an obtained compositematerial is very small to the extent of less than 5 ppm which issubstantially the same as purity value of the matrix on condition that ahigh pure boron compound is employed. This is because pollution possibleto occur in the prior art during the mechanical steps of grinding,mixing compressive moldings does not take place at all.

The advantage of the method according to the invention is typicallyexhibited when boronizing carbon/carbon composite material. In theconventional method employing B₄ C powder, several specified equipmentsand techniques are needed for grinding to the particle size of less than1 μm, and in which obtained fine particles are admixed with resincomponent, applied to carbon fibers, thus making a prepreg, which isthen molded, hardened by heating and carbonized, finally obtaining aboronized C/C material. A serious problem, however, exists in that thecarbon material cannot be completely graphitized. Because, despite thata high temperature of 2500° to 3000° C. is required for graphitizationof carbon, B₄ C component begins decomposition at about 2300° C.Accordingly, it is almost impossible to insert fine B₄ C particles intothe fine pores of the C/C material preliminarily graphitized by bakingat a high temperature of 3000° C. Even distribution of boron componentup to the deep portion of the C/C material is difficult all the more.The situation is same in case of ordinary carbon black. As for the C/Cmaterial, however, there is a peculiar difficulty in achieving requiredboronization while keeping strength of carbon fibers.

On the contrary, boronization of C/C material can be performed veryeasily. As mentioned above, boron component can be forcibly press-fitted(compressively inserted) into fine pores of the carbon materials in theform of melt or solution in molecule size, and dispersed evenly up tothe deep portion. Furthermore, there is no change in organization of theC/C material as a result of forced press-fit (compressive insert) ofboron component and subsequent baking thereof, and carbon material ispreliminarily graphitized at 3000° C. As a result, even when baking at2000° C. to accelerate reaction of boronization, a compact of boron thusobtained has satisfiable physical property to be qualified as a C/Cmaterial.

In the meantime, any boron compound can be employed as a boron componentto be impregnated into carbon material for achieving the object of theinvention as long as the boron compound can be transformed into asolution by thermal melting or by solvent. However, any boron compoundwhich leaves mineral impurities after being baked together with carbonmaterial is not desirable because of inviting pollution of carbonmaterial eventually resulting in restriction on the use. In other words,desirable is a compound which is either thermally decomposed by bakingor decomposedly volatized by reaction with carbon leaving boron alone.From this viewpoint, it may be said that organic compound containingboron, boron halide, etc. are desirable. But from the viewpoints ofeconomy and easy handling, boron oxide (B₂ O₃) and hydrate compound suchas H₃ BO₃ ortho-boric acid can be said most suitable. For example, withregard to chemical reaction between B₂ O₃ and carbon, a following bakingreaction of B₄ C is well known:

    2B.sub.2 O.sub.3 +7C→B.sub.4 C+6CO

It is, however, not always clear whether or not reaction of compositematerial (carbon-boron), which has been produced in such a manner thatB₂ O₃ of molecule size in a large amount of carbon as is done in theinvention, follows exactly and smoothly the above mentioned reactionformula. In this respect, as a result of various analyses on thecomposite material obtained by the method shown in the later-describedExample 1, 4% by weight of boron component (free B₂ O₃ 0.02%) weremeasured depending upon chemical analysis. Further, as a result ofobservation by X-ray analysis, a peak value showing the existence of B₄C was not found despite that the fact of neutron absorption by the boroncomponent was clearly recognized as a result of neutron irradiation.There was no other peak showing any other crystal system, though largebroad parts were shown, hence it seems that the composite material is inan indefinite form or in a form of solid solution. Accordingly, it issupposed that final product obtained is not in a definite form showing aspecific compound of B₄ C, but is in a form of solid solution of(B×Cy+C), though the present invention is not restricted to the form ofsuch solid solution.

Boron oxide (B₂ O₃) as well as hydrate thereof can be used as the boroncomponent in the invention. Boric acid (H₃ BO₃, B(OH)₃) is one of suchdesirable hydrates, for example.

The mentioned boric acid has a relatively low melting point (185° C.) ascompared with boron oxide (B₂ O₃). Under the temperature higher than thementioned, boric acid is decomposed while emitting H₂ O, and transformedinto a solid solution (B₂ O₃ ·^(n) H₃ BO₃) to be kept in a state like aliquid. In this manner, when employing a boric acid as a startingmaterial, the boric acid is kept to a temperature having a moderateviscosity in the container, i.e., at 300°to 500° C. while melting boricacid, then a carbon material is dipped therein and forcibly press fitted(compressively inserted) into fine pores of the carbon material. Thesecond process (HIP treatment) following the mentioned first process(impregnation) can be performed in the same manner as the foregoing B₂O₃.

Described hereinafter is an embodiment of the method according to theinvention in which the mentioned boron compound and carbon material areemployed as starting materials.

The boron compound melts by heating and is impregnated under pressure ina state of liquid or solution dissolved into an adequate solvent. Forexample, melting point of B₂ O₃ is 450° C. under normal pressure,boiling point is 1500° C. and liquefied in this temperature range of450° to 1500° C. Temperature range for the impregnation is 600° to 1400°C., preferably 800° to 1200° C.

In the first process, B₂ O₃ and carbon material are placed in a pressurecontainer, then the B₂ O₃ is press-fitted (compressively injected) infine pores of the carbon compact by heating (vacuum) method underpressure. In this step, press-fit (compressive injection) of B₂ O₃ iscompletely and easily performed by eliminating, before the press-fitting(compressive injection of) B₂ O₃, air existing in the fine pores of thecarbon material by temporarily reducing pressure in the container.However, this temporal pressure reduction is not always essential sincepress-fit pressure is high. The press-fit pressure may be severalkg/cm², preferably 50 to 100 kg/cm².

In the second process, HIP treatment is performed. Even if the carbonmaterial impregnated with boron compound in the first process is heatedat 2000° C. under normal pressure, the carbon material is hardlyboronized. This is because boron compound is evaporated by heating athigh temperature, there is no room for solid solution by reaction withthe carbon material. The heating in this second process is thereforerequired to be performed under pressure. For example, employing an inertgas such as Ar to be a medium, the heating should be performed under thepressure of not lower than 100 kg/cm² and at a temperature not lowerthan 1500° C., preferably 100 to 2000 kg/cm² and not lower than 2000° C.Boron compound is dissolved, dispersed and fixed to the carbon materialby this HIP treatment.

The mentioned second process is as essential as the preceding firstprocess. To get ordinary solid solution of carbon-boron, necessarycutting is performed at this stage to be put on sale in the market.

However, with respect to a material possible to be used in a specificuse such as neutron absorbent for reactor, it is preferable thatresidual amount of B₂ O₃ is as small as possible. When placing such B₂O₃ in a reactor and used at high temperature, B₂ O₃ evaporated isprecipitated and solidified at relatively low temperature portions,which may result in a problem, i.e., an obstacle of required reactionotherwise bringing about corrosion of metal parts.

To meet the situation, the following third process may be added toeliminate a small amount of B₂ O₃ still remaining in the carbonmaterial.

That is, the solid solution obtained through the second process isfurther treated under the pressure of not higher than 10 Torr,preferably not higher than 5 Torr, and at the temperature of not lowerthan 1500° C., thereby B₂ O₃ being evaporatively eliminated.

As a result of carrying out the mentioned treatment, residual amount ofB₂ O₃ can be decreased to not more than 0.1%.

The material obtained by the method according to the present inventionhaving multi-purpose property and versatility is satisfiably applicableto almost all use as superior reactor materials including oxidationresistant consumables, neutron absorbent, or as wall material facing toplasma in reactor, materials for aerospace industry, parts formetallurgy and machinery.

Furthermore, the composite material obtained according to the inventionperforming very excellent neutron absorption, oxidation resistance ascompared with the conventional composite materials can be employed notonly as neutron absorbent, crucible for molten metal, stirring bar,liquid level detection probe, nozzle for continuous casting, die for hotpress, etc. but also in the following uses;

(1) die for continuous casting;

(2) heating element;

(3) jigs such as stirring bar for molten metal, sensor probe:

(4) graphite for reactor (composite material containing B is notapplicable as reactor core material for use in high temperature gasfurnace);

(5) bearings used at high temperature;

(6) crucibles;

(7) casting molds for hot press;

(8) high jigs including post chips; and

(9) carbon material for hermetic seal

In the composite material thus obtained according to the invention,boron is evenly diffused or distributed in carbon. Further, carboncompact thereof can be selective among any kind including C/C material,high density graphite, etc. and any shape. In effect, it may be saidthat a carbon boron composite material having never been obtainedaccording to the prior art can be now obtained in the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic explanatory views prepared both based onphotographs taken by exposure to neutron irradiation;

FIG. 3 is a graph showing percentage of oxidation exhaustion of variouscarbon materials;

FIG. 4 is a schematic view for explaining a manner of simulation test ona crucible for use in glass molding;

FIG. 5 is a schematic view for explaining a manner of durability test ona heating element; and

FIG. 6 is a schematic view for explaining a manner of durability test ona cylinder;

DESCRIPTION OF PREFERRED EMBODIMENT EXAMPLE 1

First Process

An isotropic graphite ("IG-11" produced by Toyo Tanso) was dipped in B₂O₃ (extra fine grade reagent) dissolved at 1200° C. using an autoclave,then a pressure of 150 kg/cm² was applied to the dipped graphite usingN₂ gas, thereby B₂ O₃ being impregnated into pores of the graphite.

Second Process

After completing the impregnation process, boron was diffused in thegraphite by means of a HIP treatment device at a temperature of 2000° C.and under a pressure of 2000 kg/cm² for 1 hour (employing Ar as pressuremedium), thus a solid solution of graphite being obtained. In thisrespect, for carrying out the HIP treatment, the product to be treatedwas put in a cylindrical sheath of graphite on which a cap was applied.

Third Process

Thereafter, using a vacuum container, a vacuum treatment was performedat 2000° C. under 1 Torr for 1 hour. Boron concentration of the obtainedcomposite material was measured by mannitol method by which it wasrecognized that the boron concentration was 4.0% by weight (with respectto boron element), among which B₂ O₃ was 0.02% by weight. This meansthat almost all of B₂ O₃ not reacted was evaporated and eliminated.

EXAMPLE 2

The carbon-boron composite material obtained through Example 1 wasfurther treated by repeating the same manner as Example 1. Boronconcentration of the boron composite material thus obtained was 7% byweight, among which B₂ O₃ was 0.03% by weight.

As apparent from the above description, it was acknowledged that contentof boron in the composite material was increased.

EXAMPLE 3

A plain weave cloth of PAN high strength carbon material (3000filaments, 7 μm in fiber diameter, 300 kg/mm² in tensile strength) wasimpregnated with a phenolic resin solution (prepared by diluting a resoltype phenolic resin with methanol to 1/2 to 1/3), and after drying itfor 24 hours, a prepreg sheet was obtained.

This prepreg sheet was laminated in a dryer, heat-treated (at 100° C.for 0.5 hour), then placed on a die and treated by holding it in ahydraulic press at 140° C. under 50 kg/cm² for 1 hour. Thus, a 2Dcompact comprising two sheets of laminate were obtained.

The compact thus obtained was inserted into coke powder and heat-treatedup to 1000° C. under non-oxidation atmosphere at a temperatureincreasing speed of 10° C./hour, then further treated using a vacuumfurnace up to a high temperature of 2000° C. under a reduced pressure of5 Torr at a temperature increasing speed of 100° C./hour. As a result, a2D C/C composite material free from crack was obtained.

A solution prepared by adding 1 part by weight of H₂ O to 1 part byweight of orthoboric acid (H₃ BO₃) was added to the mentioned 2D C/Ccomposite material to be dipped and impregnated thereinto. H₂ O of thecomposite material was then evaporated in a dryer kept at 120° C.Thereafter, another impregnation treatment with an aqueous solution wasfurther carried out. It was recognized that the aqueous solutionobtained was relatively low in viscosity and impregnated easily intodeep portion through the gap and fine pores.

After completing the mentioned first process (impregnation treatment),the second process was carried out on the same conditions as theforegoing Example 1, and a carbon-boron composite material mainlycomprising of a matrix of C/C composite material was obtained. Boronconcentration of the product thus obtained was 3.7% by weight (valueconverted to boron element).

EXAMPLE 4

A mesophase spherical carbon ("KMFC" produced by Kawasaki Steel Corp.)was ground to fine particles of not larger than 5 m in average grainsize. After thermo-compressive molding, the fine particles were baked at2500° to 3000° C., and using the obtained highly pure ultrafineisotropic graphite material (hereinafter referred to as ISO-880), areaction of boronization was performed in the same manner as that ofExample 1.

This carbon matrix was a carbon material of high strength and of whichfine particle's capacity is small. Thus as a result of performing aboronization in the same manner as the foregoing Example 1, it wasrecognized that concentration of boron in the obtained C-B compositematerial was 2.6% (by weight) and residual amount of B₂ O₃ after thethird treatment was not more than 0.01%.

In addition, Table 1 shows values obtained by analyzing the ISO-880material and the elements other than boron after the reaction ofboronization.

                  TABLE 1 (1)                                                     ______________________________________                                        Element   Al     As     B   Be   Bi   Ca   Cd   Ce                            ______________________________________                                        Purity level                                                                  ISO-880   <0.3   --     --  --   --   --   --   --                            ISO-880 + B                                                                             <0.3   --     (*) --   --   --   --   --                            ______________________________________                                    

                  TABLE 1 (2)                                                     ______________________________________                                        Element   Cr    Cu     Fe   Ga   Ge   Hg   In   K                             ______________________________________                                        Purity level                                                                  ISO-880   --    --     <1.0 --   --   --   --   --                            ISO-880 + B                                                                             --    --     <1.0 --   --   --   --   --                            ______________________________________                                    

                  TABLE 1 (3)                                                     ______________________________________                                        Element  Li     Mg     Mn   Ni  P    Pb  Si   Sn  Ti                          ______________________________________                                        Purity level                                                                  ISO-880  --     <0.1   --   --  --   --  <0.1 --  --                          ISO-880 + B                                                                            --     <0.1   --   --       --  <0.1 --  --                          ______________________________________                                    

Though ordinary carbon material usually contains about 400 ppm ofimpurities, this impurity value can be decreased to not more than 10 ppmby halogenation treatment at high temperature (as disclosed in theJapanese Laid-Open Patent Publication (unexamined) No. 63-79759), and itis further possible to reduce total quantity of ash to 1 to 2 ppm whenrequired. It may be said that the ISO-880 in this example is a materialobtained by eliminating impurities beforehand using the halogenationtreatment disclosed in the Japanese Patent Publication No. 63-79759, forexample. Atomic absorption analysis and bright line spectrum analysis orthe like were adopted together as analysis method. In the table, (-)indicates elements not detected.

As is explicit from the result of analysis of impurity amount before andafter the treatment of boronization, increase of elements other thanboron is not found.

EXAMPLE 5

A test sample was prepared employing IG-110 as a matrix, said IG-110being obtained by highly purifying IG-11 serving as carbon material inthe same manner as Example 4 (content of boron of this sample was 4.2%).

Other test samples were prepared by the methods shown in Examples 1 and3, and these samples were all subject to neutron irradiation test toacknowledge the manner of diffusion of boron element.

For such acknowledgment of diffusion of boron, the inventors tookadvantage of the property of boron having very high neutron absorptionperformance.

Described hereinafter are results of the mentioned recognition ofdispersion or diffusion of boron in the test samples with the use ofneutron irradiation method.

The neutron irradiation test was performed on the test samples preparedin Examples 1 and 3 as well as on the sample prepared according to theprior art.

Test Samples Employed

Test Samples Prepared According to Prior Art

B₄ C powder available in the market was ground and those of 3 to 7 μm ingrain size were selected to be employed as test sample. On the otherhand, 50 parts by weight of coke powder (not larger than 15 μm inaverage grain size), 10 parts by weight of artificial graphite powder(not larger than 40 μm in average grain size) and 40 parts by weight ofpitch were admixed together and kneaded while heating (at 230° C. for 2hours), then molded and fine ground. Thereafter 7.7 parts by weight ofthe mentioned B₄ C particles were added to 100 parts by weight of theproduct obtained by the mentioned fine grinding, then heated and kneadedtogether with a small amount of caking agent. The kneaded product wasfurther molded under pressure, and baked at 2000° C., thereby a testsample being obtained. As a result of chemical analysis, content ofboron was 4.2% by weight (Value converted to pure boron element).

Each of the three test samples obtained as described above were then cutto be a thin plate of 2 mm in thickness, and subject to neutronirradiation test on the following manner:

Neutron irradiation testing device:

Neutron Radiography produced by Sumiju Test & Inspection.

Beam irradiation quantity:

34.4 μA 4653 sec (160.0 m Cb)

Neutron irradiation method:

Each test sample was put on a dry plate and irradiated with neutron.Portions where neutron was absorbed became white, while portions whereneutron was not absorbed were blackened by exposure.

Test Result

FIGS. 1 to 2 show test results. These drawings are schematic explanatoryviews illustrated based on photos taken by exposure to neutronirradiation.

As for the product according to the prior art, boron compound was foundexisting in the form of B₄ C fine particles, and portions where neutronwas absorbed remained in the form of white spots as unexposed parts. Onthe other hand, portions without boron (i.e., irradiated by neutron)remained blackened by exposure. The drawings show the mentioned spots of10 magnifications to clearly show them.

In the case of Example 1, it is recognized that boron components arevery finely and evenly diffused. No white spot is found no matter howenlarging the original photo picture. Accordingly, the exposure resultedin showing an even intermediate color between white and black on allover the picture in the drawings, and no white spot was found beingdifferent from FIG. 1.

As mentioned above referring to FIG. 1, despite that 4% boron compoundexisted actually, no portion of white dots showing absorption of boronwas presented. This means that boron was diffused in the form of veryfine particles.

In the case of Example 3, boron was impregnated into a carbon-carboncomposite material. Though no result of analysis in the form ofphotograph was prepared, it is understood that boron is ultrafinelydistributed evenly throughout the entire test sample.

As is understood from the above-discussed comparison between the productobtained according to the prior art and that obtained according to theinvention, there is a remarkable difference in the aspect of borondispersion or diffusion therebetween. Thus, in the present invention, itis obvious that boron is evenly diffused in such a manner as to be fineincomparable to B₄ C particles according to the prior art.

Substantially the same results as above were obtained also with regardto Examples 2 and 4.

In addition, the C-B composite material wherein boron component is ultrafinely diffused has a superior oxidation resistance. Such oxidationresistance is an essential requirement in the event of using a carboncomposite under oxidizing atmosphere. In this respect, describedhereunder is an example of measurement of oxidation resistance of thetest sample prepared by the method according to the invention:

EXAMPLE 6

Oxidation resistance of the following (boron-carbon) composite materialprepared by the methods in Examples 1 and 5 was analyzed.

COMPARATIVE EXAMPLE 1

The test sample (containing 4.2% of boron) prepared by the methodaccording to prior art (employing B₄ C powder: the same one as thementioned neutron test).

COMPARATIVE EXAMPLE 2

The carbon matrix (IG-11) employed at the time of preparing the testsample used in Example 1 (containing 0.0% of boron).

COMPARATIVE EXAMPLE 3

The carbon matrix (IG-11) employed at the time of preparing the testsample used in Example 1 and further highly purified by halogenationmethod (IG-110) (containing 0.0% of boron).

Each of the above described five test samples was cut (32×20×12.5 mm),then put and left in an air bath heater kept at 700° C. Reduction inweight and percentage of oxidation loss of each test sample was measuredat appropriate time intervals. FIG. 3 shows the results of suchmeasurement: where reference symbols respectively denote the following:

A: Example 1

B: Example 5

C: Comparative Example 1 (material according to prior art)

D: Comparative Example 2

E: Comparative Example 3

As is clearly seen from FIG. 3, in the Example 1 (hereinafter indicatedas A in the drawing), oxidation resistance was remarkably improved byimpregnation of boron component as compared with Comparative Example 2(indicated as D) in which matrix (IG-11) before impregnation with boroncomponent was employed. Moreover, it is to be noted that oxidationresistance was remarkably high when boron contents were at the samelevel, as compared with the product according to the prior method(Comparative Examples 1, C) in which B₄ C powder was added.Substantially the same result was recognized through the comparisonbetween Example 5(B) in which treatment of boron addition was applied toa highly purified material and Comparative Example 3.

These results seem to come out by the following reason. That is, in theprior method, boron component which is a B₄ C powder expected to performoxidation resistance effect exists partially in the form of grains, andthere are more portions without boron component from where oxidationbegins. On the other hand, in the present invention, since boroncomponent is very finely distributed throughout the entire part evenly,reaction of oxidation tends to be restrained as a whole.

For producing a C-B composite material of the present invention, it isto be noted that boronizing reaction of carbon material according to themethod of the invention is featured by accomplishing even and ultra-finediffusion of boron. A further feature exists in that the boronization isapplicable to any kind and shape of carbon material without negativelyaffecting nature and physical property of object material.

In this sense, the foregoing Table 1 shows comparison between physicalproperty before boronization of carbon material employed in theinvention and that after boronization thereof. In addition, followingTable 2 shows Comparative Example 4.

COMPARATIVE EXAMPLE 4

The test sample prepared by the method according to Example 4 and notboronized at all.

                  TABLE 2                                                         ______________________________________                                               Comp.             Comp.                                                       Ex. 4  Example 4  Ex. 2     Example 1                                         ISO-88 ISO-88 + B IG-11     IG-11 + B                                  ______________________________________                                        Bulk specific                                                                          1.90     1.95       1.77    1.86                                     gravity                                                                       (g/cm.sup.3)                                                                  Bending  950      950        400     400                                      Strength                                                                      (kg/cm.sup.2)                                                                 Elastic  1300     1300       1000    1000                                     Coefficient                                                                   (kg/mm.sup.2)                                                                 Thermal  70       70         116     116                                      conductivity                                                                  (W/mK)                                                                        ______________________________________                                    

It is understood from the above Table 2 that, as a result of applyingboronization, neither organization, structure, etc. of the originalcarbon material nor physical property thereof remains unchanged.

EXAMPLE 7

Simulation test on a crucible for glass molding was carried out.

Flat molds (1) shown in FIG. 4 were prepared respectively using "IG-11"(Comparative Example 2), Example 1 and the mentioned product accordingto prior art. Each molten boro-silicate glass (2) is injected into themold (1) at 1300° C. under natural atmosphere, then solidified bynatural cooling, and the solidified glass was taken out. This processwas repeated to determine durability.

In the test, thickness of the mold was made thin for the purpose ofobtaining test result in short time, and durability was determined bycounting number of repetitions up to destruction of the mold due torepetitions. To be more specific, in the flat mold in FIG. 4, "d" is 50φmm, "h" is 30 mm, and thickness is 3 mm.

    ______________________________________                                                         Number                                                       Material         of times   Sample                                            ______________________________________                                        Comp. Example 2 (IG-11)                                                                        5          Comp. Example                                     Material by prior method                                                                       8          Comp. Example                                     Example 1 (IG-11 + B)                                                                          15         The invention                                     ______________________________________                                    

As is seen from the above table, the carbon material according to themethod of the invention exhibited a satisfiable durability as comparedwith the original matrix as a matter of course and with the carbonmaterials prepared by the prior method.

In another test on the same glass to which hermetic seal was applied,the invention was more effective than the product according to the priorart.

EXAMPLE 8

When using a carbon material as a heat element under the atmospherecontaining large amount of oxygen, carbon dioxide gas, moisture and thelike, there usually arises a problem of deterioration due to oxidationexhaustion. Hence, it is preferable that even heating and partialheating in the heating element are as small as possible. From thisviewpoint, the boron-carbon composite material according to theinvention in which boron is evenly and finely diffused in graphitematrix is particularly effective.

Then, a heating element was prepared of the same material as Example 7,and subject to a deterioration test under the atmosphere using themethod illustrated in FIG. 5 as follows:

    ______________________________________                                        Material         Hour (h)   Sample                                            ______________________________________                                        Comp. Example 2 (IG-11)                                                                        0.5        Comp. Example                                     Material by prior method                                                                       1          Comp. Example                                     Example 1 (IG-11 + B)                                                                          2          The invention                                     ______________________________________                                    

Measurement conditions:

under atmosphere;

800° C.

measured was a time when electric current value was sharply reduced fromnormal current value due to imperfect contact at bolted part because ofdeterioration.

It is understood from the above result that the carbon material preparedby the method of the invention has a superior durability also whenemployed as a heating element.

EXAMPLE 9

Forming a graphite containing boron into a cylinder for use in air hotpress and employing an isotropic high density graphite as a punch, a hotpress was manufactured, and a test was carried out on the life ofcylinder.

In this test, Al₂ O₃ powder was heat-treated at 1400° C. (for 2 hours)under 180 kg/cm². Cylinders were respectively prepared using the hotpress of the same material as Example 7. Durability of each cylinder wastested by the method shown in FIG. 6.

    ______________________________________                                                              Number                                                  Material              of times                                                ______________________________________                                        Comp. Example 2 (IG-11)                                                                             5                                                       Boron material by prior method                                                                      8                                                       Example 1 (IG-11 + B) 15                                                      (the invention)                                                               ______________________________________                                    

Referring to FIG. 6, lower part of the cylinder (21) was filled withalumina powder (23) interposing a separator (22), and another separator(24) was placed on the alumina powder. Then a pressure was applied by apunch (25) in the direction of the arrow, and number of times ofrepeated use showing a durability was measured.

What is claimed is:
 1. An oxidation resistant carbon material includinga composite material mainly composed of a carbon and boron, saidcomposite material being obtained by impregnating a carbon material witha boron oxide and/or a hydrate compound thereof; and baking such carbonmaterial under pressure by inert gas at a temperature of not lower than1500° C., wherein the total amount of mineral impurities other thanboron compound is not larger than 20 ppm and the amount of residual freeB₂ O₃ is not larger than 0.1% by weight.
 2. An oxidation resistantcarbon material as set forth in claim 1, wherein said carbon material isa high density isotropic graphite material.
 3. An oxidation resistantcarbon material as set forth in claim 1, wherein said carbon material isa carbon-carbon composite material reinforced by carbon fibers.
 4. Anoxidation resistant carbon material as set forth in claim 1, whereinsaid impregnating is conducted at a temperature of 600°-1400° C. and apressure of at least 50 kg/cm².
 5. An oxidation resistant carbonmaterial as set forth in claim 1, wherein said impregnating is conductedat a temperature of 800°-1200° C. and a pressure of 50-100 kg/cm².
 6. Anoxidation resistant carbon material as set forth in claim 1, wherein thebaked material is further treated under a pressure of not higher than 10Torr and at a temperature of not lower than 1500° C. for a length oftime to remove substantially all unreacted boron-containing compounds.